1. Field of the Invention
The invention relates to a cap for the hip head or femur head for endoprostheses. The cap has an essentially spherical shape and has a recess for partially receiving the femur.
2. Description of the Prior Art
The use of spherical caps at the proximal end of the femur is known and is described, for example, in British Pat. Nos. 720,092, 764,600, U.S. Pat. Nos. 3,521,302, 3,925,824, German Offenlegungsschriften Nos. 27 24 040, 25 12 407, 25 35 649, 24 22 617, and German Pat. Nos. 1,164,019, 923,383.
It is common in this art that for insertion, work must be performed on the entire femur head. In most cases, this work essentially destroys the supply vessels of the neck of the femur, i.e., the blood supply to the bone stump is no longer ensured. This causes necroses of the stump and, as a result, a loosening of the spherical cap.
Another significant difficulty with the previous fastenings of the spherical caps resided in preventing their rotational movement. For example, it has already been suggested to provide vertical ribs or grooves in the spherical cavity, see British Pat. No. 720,092, and U.S. Pat. No. 3,925,824, or to achieve securement through fastening pins which simultaneously are supposed to serve as the primary anchoring, see U.S. Pat. No. 3,521,302, German Pat. No. 1,164,019, and German Pat. No. 923,383.
In other caps, the fastening was effected by means of bone cement. However, the tissue is severely damaged by the heat load occurring during hardening. Also, toxic phenomena caused by the monomer of the bone cement are possible.